1. Field of the Invention
The present invention relates to a semiconductor lithography system, and more particularly to a novel apparatus and method of aligning a substrate to a reticle.
2. Description of the Prior Art
In the semiconductor industry, several processing steps require the alignment of a substrate, such as a semiconductor wafer coated with a photosensitive material to a reticle having a pattern of a particular device layer. After alignment, the reticle is exposed to radiation to which the photosensitive coating is sensitive, to transfer the reticle pattern onto the wafer. This alignment and exposure can be performed on a variety of lithography systems such as step and repeat, projection, contact and proximity systems, for example. Typically, the first of such device layers is aligned to some marking on the wafer, for example, to a flat or notch, as is well known. Subsequent layers are then aligned relative to this first layer and/or to each other.
Most systems utilize some mechanical means of pre-aligning the wafer, so that the wafer is coarsely aligned to the reticle. The pre-alignment may be, for example, a mechanical means of locating a flat or notch on the wafer. Alternatively, optical sensors may determine the location of the flat, notch, or peripheral edge of the wafer. These methods typically align the wafer to an accuracy of a few hundred microns. After mechanical pre-alignment, the wafer is moved to or near the exposure position by, for example, a moving arm. Often, after the above-described mechanical alignment and prior to fine alignment, a pre-alignment using the photoelectric detector described below is performed. Special optical alignment targets (OATs), produced on the substrate by previous processing steps, are used for this purpose. The OATs are relatively large, so that they can be quickly found after the relatively coarse mechanical pre-alignment. This pre-alignment using the OATs typically aligns the wafer to within approximately .+-.50 microns or better. At this point, a fine alignment may be performed, by aligning alignment keys on the reticle to alignment targets on the wafer.
The alignment keys and targets are typically on the order of a few microns in size, and provide for alignment to a precision of, for example, 0.15 micron or less, depending upon the requirements of the user. The fine alignment can be performed via a photoelectric detector, such as a photomultiplier tube, which can detect the superposition of special purpose alignment marks on the reticle and wafer. Based upon the superposition signal level, the detection apparatus sends a signal to move the stage such that the alignment targets on the wafer are in alignment with the alignment keys on the reticle. The system may be an off-axis system, wherein the wafer is aligned out of the exposure of field of the optical system, then moved to the exposure field with high accuracy to align the wafer to the reticle. Alternatively, the alignment can be performed through the lens (TTL) of the optical system. Some off-axis systems rely upon aligning the reticle to a mechanical reference built into the lithography apparatus. The substrate is aligned to this mechanical reference as well, and thus to the reticle by commutation. However this scheme requires that the mechanical reference be frequently calibrated for the offset of the substrate to the reticle. Furthermore, very high mechanical stability is required. The TTL schemes allow the examination of the actual superposition of reticle image and the substrate for alignment, thereby eliminating the need for a mechanical reference. A TTL scheme can be configured in a variety of ways, for example, the prior art method of using the lens to view the projection of the reticle image onto the substrate. Many alignment systems require scanning, that is, relative motion between the reticle and wafer, which introduces some error.
One problem with prior art alignment systems is that the pre-alignment OATs require a relatively large area of the wafer. Often, an area equal to that of two entire die is sacrificed for this purpose, thereby reducing the possible yield of the wafer.
An additional problem in the prior art is that each type of lithography system requires that its own specially designed alignment targets be placed on the wafer in order to align the layer being exposed to a previous layer. Thus, it is often impractical to "mix and match" different lithography systems. For example, in a fabrication facility where several different types of equipment are used, a significant amount of area must be utilized to place all the necessary alignment targets on the wafer. In addition to taking up extra space in the die area, this makes it very difficult to substitute equipment as a certain lithography system cannot be used for an alignment step if the appropriate alignment target is not present. Thus, if a the particular lithography system which is used for a given step breaks down, an otherwise available lithography system cannot be used in its place. An additional problem is that if an alignment key on the reticle or alignment target on a wafer is missing or obscured, there is no way to align the wafer to the reticle.
In some applications, such as thin film head manufacture, there is no way to incorporate cooperative alignment targets between some layers because the piece part is cut up and rotated through 90 degrees between layers. In these situations, a human operator is required to align the piece part to the reticle, decreasing throughput and increasing costs.
What is needed is a system which allows for pre-alignment without the use of large OATs. What is further needed is a system which provides for precise fine alignment, without requiring that special alignment targets be placed on the wafer for that purpose. Such a system should be capable of providing for mix and match type operation such that it could be used to align a reticle to a wafer regardless of the system or systems used for the preceeding layers. Furthermore the system should be capable of aligning a wafer to a reticle, even where any predetermined alignment keys or targets have been removed or obscured. Finally, the system should provide for automatic alignment between a reticle and a substrate even when the substrate has been cut and rotated relative to the previous layer.